1. Field of the Invention
The present invention relates to a position measuring apparatus for receiving waves from satellites of GPS (Global Positioning System) and for measuring the present position of the apparatus.
2. Background Art
Conventionally, a position measuring apparatus such as a car navigation apparatus which uses data received from GPS satellites knows substantially the exact positions of the respective GPS satellites based on satellite messages called ephemeris and almanac data carried by radio waves from the respective GPS satellites to reduce times required for catching the respective GPS satellites, or times required for synchronizing reception timings of the respective C/A (Coarse/Acquisition) codes from the satellites with transmission timings of those codes (satellite catch times) in order to achieve the position measurement in a short time.
The ephemeris data includes detailed data about an orbit of each sending satellite and correction to a clock time and is always updated, which can be achieved in about 30 seconds. The almanac data relates to the orbits of all the satellites and is effective for a long time unless the satellites malfunction, and takes about 12-13 minutes to acquire. Thus, the measuring apparatus stores in memory the ephemeris and almanac data acquired once. New ephemeris data is acquired. New ephemeris data is acquired by position measurement each time, for example, its effective term (of about 4 hours) has elapsed. Thus, as long as the ephemeris data is effective and the condition to receive radio waves is good, the position measurement is achieved generally in about 15 seconds.
However, if the ephemeris data is ineffective, the conventional position measuring apparatus is required to continue the position measurement for over 30 seconds even when the receiving condition is good in order to receive new ephemeris data. In a portable position measuring apparatus of especially a small type, its GPS antenna is small compared to that of the car navigation apparatus, and has a poor performance of receiving radio waves, which is likely to be influenced by its environment of use. Thus, when new ephemeris data is required to be acquired, a good receiving environment continuous for a predetermined time cannot often be secured, in which case thereafter the position measuring time will increase. Thus, with such a portable position measuring apparatus, the user who wishes to know his or her present position cannot often know it immediately, and the apparatus is not necessarily handy.
It is therefore an object of the present invention to provide a position measuring apparatus in which the position measurement is achieved steadily in a short time.
In order to achieve the above object, according to the present invention, there is provided a receiver for receiving radio waves from satellites, comprising:
measuring means for receiving radio waves from the satellites and for measuring a present position of the receiver;
a memory for storing ephemeris data acquired from the radio waves and used for measurement of the present position of the receiver by the measuring means;
determining means for determining a time elapsing since the ephemeris data was acquired;
reporting means for reporting to a user the necessity for acquisition of new ephemeris data; and
control means, responsive to the elapsing time determined by the determining means exceeding a preset time, for causing the reporting means to report this fact to the user.
According to another aspect of the present invention receiver, there is also provided a receiver for receiving radio waves from a satellite, comprising:
calculating means for receiving the radio waves from the satellite and for calculating a present position of the receiver;
a memory for storing data on the satellite acquired from the radio waves and used for calculation of the present position of the receiver by the measuring means;
determining means for determining a time elapsing since the satellite data was acquired;
reporting means for reporting to a user the necessity for updating the satellite data; and
control means, responsive to the elapsing time determined by the determining means exceeding a preset time, for causing the reporting means to report this fact to the user.
According to still another aspect of the present invention receiver, there is also provided a position measuring apparatus comprising:
calculating means for receiving radio waves from satellites and for calculating a present position of the apparatus;
a memory for storing ephemeris data obtained from the radio waves and used for calculating the present position of the apparatus, and a time when the ephemeris data was acquired;
registering means for registering a present time;
determining means for determining a time elapsing since the ephemeris data stored in the memory was acquired, based on the time when the ephemeris data was acquired and the present time registered by the registering means;
reporting means for reporting to the user the necessity for acquiring new ephemeris data; and
control means, responsive to the elapsed time determined by the determining means exceeding a preset time, for causing the reporting means to report this fact to the user.
In these arrangements, when the time elapsing since data such as the ephemeris data was acquired from the satellites exceeds a preset time, the control means causes the reporting means to report to the user the necessity for acquisition of the satellite data. In response, if the user performs an operation to acquire the data, the acquired data is stored in the memory. Thus, by presetting a predetermined effective time not longer than the effective duration of the ephemeris data, occurrence of the situation where the satellite data is ineffective in the position measurement is avoided.